The invention relates to a micromechanical structures that include movable elements. In particular the invention relates to an arrangement for coupling such movable elements to other structures of a microelectromechanical system (MEMS).
In microelectronics the trend has been towards a higher level of integration. The same applies to micromechanics as well. Consequently, micromechanical elements designated especially for microelectronic purposes need to be more highly integrated because of the requirement for smaller and smaller components for electrical applications.
Prior art micromechanical components have been optimized for low frequency ( less than 1 MHz) applications and used mainly for inertial and pressure sensors. The design of micromechanical RF components for 1 to 5 GHz applications used in mobile terminals sets demands on micromachined structures. These demands are partly different from the problems in the low frequency Micro Electromechanical Systems (MEMS) applications.
The optimization of the capacitive micromechanical structures is subject to several parameters:
Sensitivity to the measured value or control force (e.g., acceleration to capacitance transfer function, control voltage to capacitance transfer function),
Signal to noise ratio that depends on the several other device parameters,
Zero point stability of the device with respect to long time periods and temperature.
These optimization criteria convert into more specific device requirements when the application and especially the measurement or operation frequency is taken into account. This invention is related to the use of the micromechanical structure as a part of the high frequency application. Two different examples of such an application are:
MEMS rf components: tunable capacitors and micromechanical microrelays;
Micromechanical low noise, high sensitivity accelerometer using LC resonance as a basis of the measurement electronics;
For both these applications, there are several common requirements for the device:
Series resistance of the device must be minimized;
Series (stray) inductance of the device must minimized and repeatable;
Temperature dependence of the structure must be as small as possible; and
Parasitic capacitance must be minimized.
The prior art micromechanical structures are mostly based on silicon and polysilicon structures. The polysilicon has good mechanical properties and technology to build suspended structures from it is well researched. However, the main disadvantage of these structures is the high series resistance. The series resistance reduces the Q value of the component at high frequencies.
Many devices like the low-noise rf voltage controlled oscillators (VCO) require a resonant device with high Q-factor, since the phase noise of an oscillator is proportional to 1QT2, where QT is the overall Q-factor of the resonator. High dynamic range filters also require a high Q-resonator, since the dynamic range of the filter is proportional to QT2. The quality factor within the frequency range 1 to 2 GHz is dominated by the series resistance. Previously, for instance the MEMS tunable capacitors were fabricated from the polysilicon, but the requirement for the low series resistance has forced to consider metal as the material of the structure. Metal can be for instance gold, copper, silver, nickel, aluminum, chromium, refractory metal or alloy of several metals.
In capacitive sensors the ultimate resolution of the capacitance measurement is limited by the series and/or parallel resistances of the sensing capacitance. Most of the prior art capacitive inertial sensors are made of doped monocrystalline or polycrystalline silicon, and the conductivity is limitted to relatively modest values. Furthermore, the additional series resistance due to metal/silicon interfaces increases the series resistance. The inertial sensors based on metal structures have been studied, [1] and [2], because of two clear advantages: 1) metals have higher material density that increases the mass and thus the sensitivity of the capacitive sensor, and 2) metals have higher electrical conductivity that reduces the electrical noise of the capacitive sensor. One of the key problem in using metallic materials for inertial sensors has been the elimination of thermal stress caused by the mismatch of the thermal expansion coefficients between the substrate and the structure.
Metal thus has some disadvantageous characteristics like the built-in stress that can cause warping of the suspended structures. In addition, most metals that are available in the MEMS processes have the thermal expansion coefficient that is very different compared to the thermal expansion coefficient of most substrate materials such as silicon, quartz or borosilicate glass. Thermal stress of the suspended structure, due to the thermal expansion mismatch, can cause severe thermal dependence in the device.
FIG. 1 shows a typical micromechanical bridge. The requirement is to make a mechanically ideal anchor using a minimum of process steps. A simple process is advantageous in the method shown in FIG. 1. One disadvantage of such metal structure is that the built-in stress and any temperature dependent stress tend to bend the suspended structure.
FIG. 1 illustrates the situation when the micromechanical metal structure with a movable element 110 and anchors 130, 132 is deposited on top of the silicon substrate 150. The FIG. 1 also shows the insulating layer 160 and the fixed electrodes 140, 142 on the substrate. The change of the internal stress of the metal 110 due to the change in temperature can be calculated as
xcex94"sgr"=Exc2x7(xcex12 xe2x88x92xcex11)xc2x7xcex94Txe2x80x83xe2x80x83(1)
where E is the Young""s modulus, xcex11, and xcex12 are the thermal expansion coefficients of the metal film and the silicon substrate, respectively, and xcex94T is the temperature change.
For the copper film on top of the silicon substrate,                                           ∂            σ                                ∂            T                          =                  2          ⁢                      xe2x80x83                    ⁢                                    MPa                              xc2x0                ⁢                                  xe2x80x83                                ⁢                                  C                  .                                                      .                                              (        2        )            
The stress in the metal causes a force Feff to the anchoring structures 130 and 132.
FIG. 2 shows the moment effect at the step-up anchor structure. We suppose that the suspended structure is connected to the substrate from several points and that the thermal expansion mismatch between the substrate and the suspended structure causes strain in the suspended structure. Effect of the strain is shown as two arrows in FIG. 2. Figure shows how the moment caused by the step-up anchor bends (exaggerated) the suspended structure. Normal dimensions for the suspended structure might be for instance that the suspended structure is 500 xcexcm long, 1 xcexcm thick and it is 1 xcexcm above the substrate. Even a very small bending moment would be catastrophic, since the structure would touch the surface.
FIG. 3 shows how the control voltage is dependent on the residual stress of a copper film double supported beam. The capacitance is kept constant, in this case 0.9 pF. Length of the beam is 0,5 mm, width is 0,2 mm, and thickness is 0.5 xcexcm. The gap between the control electrode and the beam is 1 xcexcm. The Figure shows how sensitive the control voltage is to low level residual film stress.
The temperature dependence of the capacitance can be calculated as                                           ∂            C                                ∂            T                          =                                            ∂              C                                      ∂              σ                                =                                    ∂              σ                                      ∂              T                                                          (        3        )            
The temperature dependence increases with the control voltage. For example, for 5 MPa residual stress, the temperature dependence of the capacitance can be 1%/xc2x0 C. at 1 V control voltage, and 24%/xc2x0 C. at 3 V control voltage. If the device is operated at low control voltages, the residual stress of the film must be minimized. At this range, the temperature dependence must be minimized by some structural modifications.
The temperature dependence has been reduced by using flexible spring support for the structure. Such prior art solutions for implementing micromechanical components are described e.g. in documents [3]-[6]. However, the problem of these prior art devices is: 1) too high series resistance, 2) too high temperature dependence, 3) too high stray inductance.
Prior art micromechanical structures comprising movable elements have therefore disadvantages related to the requirements described above. The prior art structures suffer from temperature dependence, due to the mismatch of thermal expansion coefficients of the micromechanical structure and the substrate. Series resistance and parasitic capacitance are also high in prior art RF components such as tunable capacitors and resonators based on a tunable micromechanical capacitor and an integrated inductor. These factors may lead to high losses, thermal unstability and unreliability of the micromechanical components.
The purpose of the invention is to achieve improvements related to the aforementioned disadvantages. The invented arrangements for coupling a movable element to other micromechanical structures facilitates minimizing the temperature dependence, the series resistance, the stray inductance and the parasitic capacitance. Hence, the invention presents a substantial improvement to the stability and reliability of the micromechanical componets, especially in the RF applications.
An arrangement according to the invention for coupling a movable element, which has a characteristic movement direction, to a fixed structure, such as substrate, of a micromechanical component, is characterized in that the arrangement comprises at least one coupling means for coupling the movable element to the fixed structure, and at least one flexible means for allowing different thermal expansion between the movable element and the other structure in a direction which is substantially perpendicular to the characteristic movement of the movable element, wherein said coupling means and/or flexible means is reinforced to be substantially inflexible in the direction of the characteristic movement of the movable element.
The invention also relates to a micromechanical component wich comprises an arrangement described above.
Preferred embodiments of the invention are described in the dependent claims.
One idea in implementing this invention is to use an additional layer, such as a metal layer, to form boundary conditions that are as close to ideal as possible for suspended structures. The inventive concept can most advantageously be realised using one or several of the following details:
1) The deflecting metal thin film is mechanically decoupled from the substrate and consists of:
a) Membrane, diaphgram or thin metal film of any shape,
b) Surrounding frame that can be of any shape as long as it is symmetric about the axes formed by two opposing anchors,
c) Inner springs that connect the deflecting element to the frame are formed on the corners of the frame,
d) Anchoring of the frame to the substrate at the middle of the frame forming beams,
e) Optional outer beams that further connect the frame and the substrate anchoring. The structure is further characterized by the symmetry shown in FIG. 9A (described in more detail in the following part of the specification), and
f) Anchoring of the frame to the substrate is arranged to be temperature compensated.
Mechanical decoupling of the movable element achieved by the structure is almost perfect. Disadvantage of the planar structure of this preferred embodiment is, however, that the corners of the frame may warp in the direction perpendicular to the substrate plane (in vertical direction) due to the built-in (residual) stress in the frame or moving element.
2) Eliminating of the warping of the structure by having larger vertical thickness for the frame than for the moving element. Another possibility to achieve a rigid vertical structure is to use profile geometries.
The invention can be implemented utilizing new fabrication technologies that are commonly known as micro system technologies (MST) or Micro Electromechanical Systems (MEMS). These fabrication technologies enable the fabrication of movable structures on top of the silicon wafer or any other substrate material. The preferred process is based on the deposition of a sacrificial material layer (silicon dioxide or polymer film) under the movable structure during the fabrication. During the final steps of fabrication the movable mechanical structure is released by etching the sacrificial layer away.
Invention improves the prior art devices (metal film structures on top of silicon substrate) in several ways:
Thermally induced stress of the deflecting thin film is minimized, below 0.5 MPa level, because of the geometrical symmetries;
Series resistance is low, below 0.1 xcexa9, because of eight parallel current paths from the thin film to the anchor;
Series (stray) inductance is low, below 0.1 nH, because of eight parallel current paths from the thin film to the anchor;
Low control voltage level possible (3-5 V) because of the low film stress; and
Warping of the mechanically decoupled structure is small.
Removes almost all the stress issued to the suspended structure due to the thermal expansion mismatch.
Relaxes the built-in stress in the suspended structure.
Series resistance of the spring structure is smaller than in the previous spring structures.
Very rigid structure in other degrees of freedom. Rigid boundaries prevent warping and allow bigger capacitors to be made, than previous structures.
Eliminates the moment effect caused by thermal deformation of the thick anchoring.